Commercial HVAC systems have conventionally contained “Single Duct Terminal Units” (“SDTUs”) for the purpose of providing an outlet for commercial ventilation systems into the rooms of a building or other structure equipped with an HVAC system. An SDTU typically consists of the following components: 1) inlet duct, 2) flow sensor, 3) modulation damper, and 4) insulated casing.
In commercial HVAC installations, a “silencer” (or “attenuator”) is often placed downstream of an SDTU in order to attenuate the sound produced by the high-velocity air exiting the SDTU. Such silencers have typically comprised an air duct (typically from three to five feet in length) that is lined internally with insulation to attenuate the noise produced by the air flowing through the SDTU. Such internal insulation is also known as a “baffle” and is usually held in place by perforated sheet metal. The perforations in the metal allow the air traveling through the silencer to interact with the insulation material contained inside the baffle. The attenuation achieved by the silencer is due to the conversion of acoustic energy into heat energy as the air molecules inside the silencer create friction when they collide with the lined insulation.
The noise generated by an SDTU can be separated into two components: 1) noise due to the air disturbance created in the immediate vicinity of the damper blade and 2) aerodynamic noise due to the flow of air that has variable pressure regions interacting with geometry changes in the air stream. The insulation contained in silencers minimizes both sources of noise created by the SDTU,
The noise generated by a given SDTU can vary widely depending on how it is utilized in a particular HVAC system and on the configuration of the HVAC system. Similarly, the acoustic performance of a given silencer can also vary widely depending upon the configuration of the HVAC system in which it is installed. Such unpredictability of the noise that will be generated by an SDTU and the attenuation achieved by a silencer is related to what is known as the “system effect” of the HVAC system in which the SDTU and silencer are installed. For instance, the manner in which the distribution ductwork is organized in a given building installation can affect the turbulence and air pressures created inside the ductwork. This, in turn, can affect the noise level generated by an SDTU and the acoustic performance achieved by a silencer attached thereto.
The unpredictability produced by such system effects creates uncertainty when HVAC installers are selecting SDTUs and silencers for installation in a building. Manufacturers of traditional SDTUs and silencers typically test their products under artificial laboratory conditions and produce specifications as to the noise generated by their SDTUs and the noise attenuation achieved by their silencers. However, these specifications do not take into account the system effects produced by installing their products in an actual HVAC system. Thus, HVAC installers generally have only marginally reliable product specifications on which they can rely and often must utilize trial-and-error methods to choose the appropriate combination of SDTUs and silencers that will meet their needs in a particular HVAC installation.